1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more specifically to a method for manufacturing a semiconductor device in which heavy-metal element contamination in the surface of a silicon substrate is caused to diffuse into the substrate, thereby reducing the contamination concentration at the surface of the substrate.
2. Description of Related Art
Heavy metal contamination that occurs in the process of manufacturing a semiconductor device is known to cause deterioration of the electrical characteristics (such as the gate oxide film reliability and junction leakage characteristics) of the semiconductor device. In recent years, with the shrinking of feature sizes and increasing in the level of integration of semiconductor devices, gate oxide film thicknesses in MOS transistors are getting progressively thinner. Thus, even minute quantities of heavy metal contamination can cause deterioration in the initial breakdown voltage and TDDB (time dependent dielectric breakdown) characteristics of a gate oxide film. According to a report in Y. Shiramizu et al in "Extended Abstracts of the 1996 International Conference on Solid State Devices and Materials, Yokohama" (pp. 362-364 (1996)), deterioration of the TDDB characteristics of an 8-nm gate oxide film is observed even with contamination to a level of 1.times.10.sup.10 atoms/cm.sup.2 of iron, which is a typical heavy metal. According to a report in "The National Tech. Road Map for Semiconductors 1994" (Semiconductor Industry Association, 1994), in a semiconductor device with a design rule at the level of 0.18 .mu.m, it is necessary to keep the heavy metal contamination level existing at the surface of the semiconductor substrate to below 1.times.10.sup.10 atoms/cm.sup.2. With this situation as a backdrop, advances have been made in improving the clean level of semiconductor device manufacturing processes, and in silicon substrate cleaning technologies, and it is possible, using a fluid mixture of acid-hydrogen peroxide, to achieve a clean silicon substrate surface to a level of 1.times.10.sup.10 atoms/cm.sup.2.
On an actual semiconductor manufacturing line, however, because there are many metals that are used in reaction chambers, gas piping, and transporting mechanisms of manufacturing equipment, it is possible that heavy metal contamination might for some reason enter the cleaning fluid, or that heavy metal contamination can occur after cleaning and before gate oxidation.
If gate oxidation is performed in this condition, there is the problem of heavy metals being taken into the gate oxide film, this leading to a deterioration of the gate oxide film material, which results in a lowering of device reliability and a lowering of the yield.
Technology to solve such problems as noted above is described, for example, in the Japanese Unexamined Patent Publication (KOKAI) H4-68526, which describes the exposure of a semiconductor substrate to an atmosphere containing hydrogen before formation of a prescribed film on the substrate so as to clean the substrate.
This technology is shown in the process diagrams of FIG. 15 through FIG. 17.
First, as shown in FIG. 15A, a silicon substrate 1 is immersed in an aqueous solution 2 that contains a appropriate chemical substance so as to remove particles and heavy metal contamination or the like that have become attached to the silicon substrate. Next, as shown in FIG. 15B, the silicon substrate 1 is transported into an oxidation oven 3, which is filled with an inert gas such as nitrogen, so that there is no silicon oxide film growth. Then, as shown in FIG. 16A, hydrogen gas is brought into the oxidation oven, to replace the atmosphere in the oven with a hydrogen atmosphere 4, after which silicon substrate 1 is left in the oven 3, which is filled with hydrogen, for a prescribed amount of time, thereby cleaning the substrate with hydrogen. Then, as shown in FIG. 16B, an inert gas that does not react with hydrogen, such as nitrogen 5, is brought into the oxidation oven, to replace the atmosphere in the oven with a nitrogen atmosphere. Additionally, as shown in FIG. 17, oxygen gas 6 is brought into the oxidation oven 3, to replace the atmosphere in the oven with an oxygen atmosphere, after which a prescribed thermal oxide film 7 is formed on the silicon substrate 1 under prescribed conditions of oxidation.
In the above-described prior art, however, because the surface of the silicon substrate is directly exposed to a hydrogen atmosphere, a reduction reaction with the hydrogen causes an increase in the microroughness in the surface of the silicon substrate. This increase in microroughness causes unevenness in the thickness of the gate oxide film that is formed on the surface of the silicon substrate in a later gate oxide process. As a result, there is a deterioration in the breakdown voltage and reliability of the gate oxide film. Thus, the prior art method was accompanied by the problem of not being able to sufficiently prevent deterioration of device characteristics.
Accordingly, an object of the present invention is to provide a novel method manufacturing a semiconductor device that improves on the problems described above and in particular which removes heavy metal contamination from the surface without causing an increase in microroughness of the surface of the silicon substrate, that is without causing a deterioration of the surface condition, whereby subsequent oxidation can be done to form a high-quality gate oxide film.